The invention relates to the transmission of orthogonal frequency division multiplexing (OFDM) signals, and specifically, relates to a method and an apparatus for inter-carrier interference (ICI) self-cancellation and ICI cancellation and reconstruction of OFDM signals.
In wireless communication systems, signals may be transmitted in electromagnetic waves through a physical channel such as air. Due to undesirable channel effects such as, for example, multi-path reflection and multi-path fading, signals may generally be distorted when received. An orthogonal frequency division multiplexing (OFDM) system based on multi-carrier modulation has been developed to address the issue of multi-path reflection. With only one simple one-tap equalizer at the receiver, the OFDM system is able to equalize signals that may have suffered from constructive interference or destructive interference caused by the multi-path effects. Therefore, the OFDM scheme becomes a mainstream scheme in the wired/wireless communications and in the digital terrestrial broadcasting applications. Examples of the OFDM applications may include Asymmetric Digital Subscriber Line (ADSL), Power Line Communication (PLC), and Digital Audio Broadcasting (DAB) systems, and may further include Wireless Local Area Network (IEEE 802.11a/b/g/n), Wi-MAX (IEEE 802.16 series), and Digital Video Broadcasting (DVB) over terrestrial (DVB-T) and over handheld devices (DVB-H) under research and development.
To prevent an OFDM system from the multi-path channel effect, a duplicate of a section of an original useful symbol, called a guard interval (GI), having a length Tg, is cyclically prefixed to the original useful symbol, which has a length Tu with N sampling points, to form a complete OFDM symbol before OFDM signals are transmitted into the channel. When the maximum channel delay, τmax, is shorter than the length of GI, τg, the inter-symbol-interference (ISI) caused by the multi-path channel may be prevented in the OFDM receiver. The receiver then discards the GI sections and retrieves the useful symbols from the received symbols, and compensates for the channel effect by an one-tap equalizer so as to estimate the transmitted data.
In the specifications of some OFDM systems, such as the DVB-H and IEEE 802.16, it is emphasized that receivers have to provide desirable capability of reception when moving at a high speed. However, when a receiver of an OFDM system is not stationary and may be mobile relative to a transmitter at a relatively high speed, the status of the channel within a useful symbol duration may be no more constant. The channel may thus become a time-selective fading channel, which may result in one time of Doppler frequency offset, either positive or negative, in carriers relative to a center carrier frequency, fc. Such a frequency offset is unfavorable to a multi-carrier modulation system such as the OFDM system, and may incur inter-carrier interference (ICI) that destroys the orthogonality of the system, disadvantageously resulting in an “error floor” effect in the bit error rate (BER).
It may be desirable to have an apparatus and a method to address the ICI issue due to the Doppler effect in an OFDM system.